The effect of titanium and nitrogen contents on the austenite grain coarsening temperature

Three series of titanium steels with different nitrogen contents were used to evaluate the effect of titanium and nitrogen contents on the grain coarsening behaviour of austenite during reheating. Quantitative metallography and transmission electron microscopy were employed to measure the austenite grain size and TiN particle size after being reheated and quenched from austenitization temperatures. The results show that for steels with the same nitrogen content the highest grain coarsening temperature occurs around the Ti/N stoichiometric ratio, i.e. 3.42 and an increase in grain coarsening temperatures with increasing nitrogen content also occurs at Ti/N around 3.42. Higher titanium content beyond the stoichiometric ratio decreases the grain coarsening temperature as a result of TiN particle growth.     

Microstructure, chemistry and coating properties of ion-plated TiN on type 304 stainless steel

Characterization of TiN coatings on type 304 stainless steel was carried out using a Zeiss EM 902A energy filtering transmission electron microscope equipped with an electron energy loss spectroscopy (EELS) detector. TiN thin films were produced by a hollow cathode discharge ion plating coater. It was found by plan-view transmission electron microscopy that the microstructure of the TiN coatings is thickness dependent. The grain size of TiN ranges from 88 nm at the coating surface down to 9 nm near the TiN/steel interface. In addition, the TiN surface layer shows some degree of texture, but the subsurface and internal TiN layers are mainly equiaxial and randomly oriented. Chemical analysis by EELS shows that the relative oxygen content increases linearly from the TiN surface to the TiN/steel interface, whereas the relative nitrogen content first decreases slowly and then drops rapidly near the interface. The presence of a Ti₂N phase and the deficiency of nitrogen near the TiN/steel interface suggest that the early-deposited TiN is nonstoichiometric. By the periodic cracking method, the ultimate shear stress at the TiN/steel interface and the residual stress in the TiN thin film were estimated to be 2.2 GPa and 12.8 GPa, respectively.     

Effects of sizes of ferrite grains and carbide particles on toughness of notched and precracked specimens of low-alloy steels

Four point bending (4PB) tests of notched specimens and COD tests of precracked specimens were carried out on two steels; one steel was treated into two groups with the same ferrite grain size but different carbide sizes, the other steel with different ferrite grain sizes but similar carbide sizes. The results of the tests show that the toughness measured in notched specimens is mainly determined by the grain sizes, which define the local fracture stress _f; the size of carbide particle plays a minor role. However, on the contrary, in precracked specimens the toughness is sensitive to the carbide sizes, which affect the critical plastic strain _pc for initiating a crack nucleus; the effect of grain size is indistinct. By these inferences the behavioral discrepancy of large grain steel in improvement of crack fracture toughness while reducing the notch toughness is explained.     

Niederenergie-IBAD: Zusammenhang zwischen Prozeßparametern und Schichteigenschaften bei ionenstrahlunterstützten Aufdampf-und Sputtertechniken

Low energy IBAD: correlation between process parameters ans film properties for ion beam assisted evaporation and sputter deposition Binary nitride films with Al, Cr and Ti as metal components have been deposited with ion beam assisted evaporation and sputtering (IBAD) and the film properties are investigated in terms of the individual deposition parameters. In the case of ion beam assisted evaporation the flux ratio between the film forming metal atoms and the nitrogen ions from the ion source was shown to enable a quantitative control of the composition and the chemical phases of the films. Detailed studies for TiN reveal the possibilities to manipulate texture and stress, the average grain size and the morphology of the films. Such results are discussed with an extended structure zone model, introducing the energy input per film forming particle as the relevant parameter. Also, the structural film properties and the deposition parameters are quantitatively correlated with the hardness and the beginning of TiN deposition on stainless steel resulted in distinctly improved adhesion properties. For the deposition of TiN with a dual ion beam arrangement in which one beam bundle was directed onto a Ti-target and an other onto the substrate with the growing film, a strong influence of the particle energies and the incidence angles on the film texture and its directional orientation was found. Such effects are quantitatively related to the minimization of the free energy of the films and the influence of preferential re-sputtering effects. For ion beam sputter deposition without simultaneous ion bombardment of the growing film, the texture and the film stress are found to be controlled by energetic particles resulting from elastic backscattering at the target surface.     

微钛低碳钢板的微观组织观察与分析

利用金相显微镜和电子显微镜等方法对微Ｔｉ低碳钢板的微观组织和第二相粒子的沉淀行为进行了研究。结果表明，基体组织由极细晶铁素体和亚晶粒组成，铁素体晶粒平均尺寸为４．８７μｍ。细小的第二相粒子在铁素体中沉淀析出，粒子的平均尺寸为１２．４ｎｍ，沉淀的粒子体积百分数为０．０３６％。在亚晶界上观察到大粒子的沉淀，位错被细小呈球形的粒子所钉扎．相分析表明，这些粒子为ＴｉＮ，Ｔｉ（Ｃ、Ｎ）或ＴｉＣ．经计算，沉淀强化与细晶强化值分别为４０ＭＰａ和２４０ＭＰａ。     

Stress-dependent deformation behaviour in bulk nanocrystalline titanium–nickel alloys

In this study, the deformation mechanisms operating with stress in bulk nanocrystalline (NC) titanium–nickel with an average grain size below a critical size of 10–20?nm have been investigated. We demonstrate a sequential variation of the deformation mechanism from grain boundary (GB) sliding and grain rotation to grain growth and dislocation activity with the increase of the deformation stress. These deformation mechanisms are different from the previous understanding that below a critical grain size of 10–20?nm, GB sliding and grain rotation govern plastic deformation of NC materials.     

Long fatigue life critical crack lengths*

A model based on surface strain redistribution and crack closure is presented for prediction of the endurance or fatigue limit stress by determining the threshold stress and critical length of short cracks that develop under microstructural control. The threshold stress first decreases with crack size to a local minimum then increases to a local maximum corresponding to the fatigue limit stress. This occurs at the critical crack length corresponding to about four grain diameters. The model is capable of determining the threshold stress range and depth of propagating and non‐propagating surface cracks as a function of stress ratio, material and grain size. The microstructure is shown to be particularly significant in the very long life regime (N_f ≈ 10⁹ cycles). When the surface cracks become non‐propagating, internally initiated cracks continue growing slowly, eventually reaching the critical crack length with failure occurring after a very high number of cycles (10⁷ < N_f < 10⁹ cycles).     

Reduction of Pd catalytic particle size by a double activation process

We have used a double activation method to reduce the size of Pd catalytic particles commonly used as an activation layer for electroless Cu deposition. The method produces Pd particles with sizes reduced by a factor of 4 and density increased by a factor of 10 compared to the single activation method. The first activation and the Pd etching process in the double activation removes the native Ti oxide on TiN surface and largely increase the nucleation sites for Pd. With more nucleation sites, nucleation events outrun ripening events throughout the deposition time range. However, excessive etching of Pd and the underlying TiN layer could lead to rougher electroless Cu films. Secondary ion mass spectrometry data show that the double activation step does not increase the net amount of Pd deposited on TiN surface; it only changes its particle size and density. Electroless Cu deposited on a doubly Pd activated surface has a larger grain size and appears to have a lower resistivity.     

Some applications of the Hall-Petch Relationship for single-phase imperfect polycrystals

In this article, the modified Hall-Petch Relationship (HPR) theory is considered further to explain an anomaly that arises in the case of nanocrystals. Strength of the material decreases as grain size decreases if the grain size is below a critical dimension. This is referred to as the “negative slope effect” in the HPR (i.e., K < 0). This effect is rationalized with the proposed revision to the HPR. A new modified HPR equation to characterize microhardness has also been derived. The new approach has been applied to films of TiN, TiC, TaC, and WxC on glass substrates and to -brass. The calculated microhardnesses of the films are compared with experimentally measured data.     

Effect of the microstructure of Si3N4 on the adhesion strength of TiN film on Si3N4

The effect of the microstructure of silicon nitride, which was used as a substrate, on the adhesion strength of physical vapor deposited TiN film on Si₃N₄ was investigated. Silicon nitride substrates with different microstructures were synthesized by controlling the size (fine or coarse), the phase ( or β) of starting Si₃N₄ powder, and sintering temperature. The microstructure of Si₃N₄ was characterized in terms of grain size, aspect ratio of the elongated grain, and β-to- phase ratio. For a given chemical composition but different mechanical properties, such as toughness, elastic modulus, and hardness of Si₃N₄ were obtained from the diverse microstructures. Hertzian indentation was used to estimate the yield properties of Si₃N_4, such as critical loads for yield (P_y) and for ring cracking (P_c). The effect of the microstructure of Si₃N₄ on adhesion strength evaluated by scratch test is discussed. TiN films on Si₃N₄ showed high adhesion strengths in the range of 80–140 N. Hardness and the P_y of Si₃N₄ substrate were the primary parameters influencing the adhesion strength of TiN film. In TiN coating on Si₃N₄, substrates with finer grain sizes and higher phase ratios, which show high hardness and high P_y, were suitable for higher adhesion strength of TiN film.     

The role of particle size on the performance of pumice as a supplementary cementitious material

A critical area overlooked in previous research on pumice is understanding how its physical characteristics influence its behavior as a supplementary cementitious material (SCM). This study investigated three pumices with different particle size distributions to observe whether these porous materials exhibit enhanced nucleation and growth of hydration products, in the same way as non-porous materials, and whether the rate of pozzolanic reaction can be changed through particle size. The effect of particle size on compressive strength, rheology and resistance to alkali silica reaction (ASR) was also evaluated. Results showed that reducing particle size increased the rates of cement hydration, pozzolanic reaction, and compressive strength gain, while also increasing mixture viscosity. Interestingly, particle size did not impact the yield stress of the mixture or the resistance to ASR. These new findings give insight about how the particle size of pumice can be used to overcome drawbacks reported in previous literature.     

Photoacoustic measurement of thermal properties of TiN thin films

Titanium nitride (TiN) thin films were prepared by reactive DC magnetron sputtering under different nitrogen flow rates and at constant substrate temperature as well as at constant nitrogen flow rate and at different substrate temperatures. Photoacoustic measurement of the thermal properties of the films revealed that the thermal diffusivity and thermal conductivity of the TiN thin films are significantly lower than the bulk values and that the grain size of the films has substantial influence on the thermal properties of TiN thin films. The thermal conductivity of the films decreases with increasing nitrogen flow rates and increases with increasing substrate temperature. The above opposing behaviour in the thermal properties is found to be related to the microstructure, especially, the grain size of the films.     

Densification model for powder compacts

The driving force of densification has traditionally been modeled on the basis of local curvature changes between powder particle pairs. Extension of particle pair analysis to powder compacts involving billions of particles has not been successful because of the geometric difference between the two cases. In this paper, a densification stress model for grain boundary and lattice diffusion controlled densification is developed on the basis of a powder compact's thermodynamics and the internal surface area evolution. For compacts with a constant grain size, the model predicts that the densification stress increases as a function of relative density, which is in agreement with experimental trends. With grain growth, the densification stress becomes relatively constant throughout the intermediate stage of densification, in agreement with experimental data in the literature. Comparison of densification rate data with densification rate model employing the developed densification stress relation also gives good functional agreement. These agreements indicate that modelling densification stress and densification rate on the basis of internal surface area captures the essential physics of powder compact densification.     

Effect of the TiN content in the Pd-TiN seed layer on the microstructure and magnetic properties of Co∕Pd multilayered media

[Co(0.2 nm)∕Pd(0.8 nm)](20) multilayered films on 15 nm Pd-TiN seed layers were fabricated by dc magnetron sputtering without heating the substrate. The effects of TiN content on microstructure and magnetic properties of the [Co∕Pd] multilayered media were studied. By increasing the TiN content in the Pd-TiN seed layer to an optimum level, coercivity of the [Co∕Pd] multilayered media increased to 6.7 kOe. However, further increase of TiN content beyond 22 vol % reduced coercivity (Hc), implying that there exists a critical TiN concentration to enhance the magnetic property of the [Co∕Pd] multilayered media. Transmission electron microscopic observations revealed that well-isolated [Co∕Pd] multilayered grains with apparent grain boundaries were achieved by controlling the TiN content in the Pd-TiN seed layer. The average grain diameter was 8 nm with a dispersion of 11.2%, grown on the Pd-TiN seed layer with TiN content of 22 vol %.     

Mode I and mixed mode fracture of polysilicon for MEMS

An experimental study was carried out to investigate the local and effective fracture behaviour of polycrystalline silicon for microelectromechanical systems (MEMS). The apparent mode I critical stress intensity factor was determined from MEMS‐scale tension specimens containing atomically sharp edge pre‐cracks, while local deformation fields were recorded near the crack tip, with high resolution by the in situ Atomic Force Microscopy (AFM)/Digital Image Correlation (DIC) method previously developed by this group. The effective mode I critical stress intensity factor varied in the range 0.843–1.225 MPa√m. This distribution of values was attributed to local (in grain) cleavage anisotropy and to enhanced grain boundary toughening. The same sources resulted in very different local and macroscopic (apparent) stress intensity factors, which, combined with the small grain size of polysilicon (0.3 μm,) were the reason for subcritical crack growth that was evidenced experimentally by AFM topographic and AFM/DIC displacement measurements. The effect of local in‐grain anisotropy and granular inhomogeneity was stronger under mixed mode loading of edge cracks inclined at angles up to 55° with respect to the applied far‐field load. The K_I–K_II locus was characterized by scatter in the K_Ic values but on average it followed the curves calculated by the maximum tensile stress and the maximum energy release rate criteria calculated assuming isotropy.     

Bonded particle modeling of grain size effect on tensile and compressive strengths of rock under static and dynamic loading

The effect of grain (particle) size on the strength is an interesting subject in the rock engineering. Some investigations about the impact of particle size on static strength of rock have been conducted and reported in the literature. However, this issue has not received enough attention when high loading rates are involved. In this work, by utilizing the CA3 bonded particle - finite element computer program, the combined influence of loading rate and particle size on the compressive and tensile strengths of rock is examined. The bonded particle model is used to simulate the crack initiation and failure of the rock specimen and the finite element is utilized to model the elastic bars in the Split Hopkinson Pressure Bar (SHPB) apparatus employed for the dynamic testing. Specimens with four different particle sizes were prepared. The results suggest that the particle size does not affect the rock strength under static and dynamic loading. However, the particle size modifies the nominal tensile strength of the notched Brazilian specimens. For the intact Brazilian specimens under high stress rates, the particle size contributes to the tensile strength and this contribution can be justified based on the principles of fracture mechanics. The theoretical reason for these observations is derived for a 3D bonded particle system and discussed.     

Conductive-atomic force microscopy study of local electron transport in nanostructured titanium nitride thin films

Simultaneous local current and topography measurements were made on the surface of titanium nitride thin films by conductive-atomic force microscopy (C-AFM). Two compositions, stoichiometric TiN and sub-stoichiometric TiN_0.76 were investigated. Local variation of current at grain and grain boundaries was examined. The current flow is filamentary in nature, with the number of percolation paths being smaller for sub-stoichiometric titanium nitride. Current-voltage characteristics of stoichiometric TiN reveal that the grain interiors are electrically conductive, while in sub-stoichiometric TiN_0.76 thin film, grains are electrically resistive, i.e., a potential barrier to electron transport exists at the junction between the grain and the grain boundary in sub-stoichiometric TiN_0.76. Therefore, electron transport in this film is due to tunneling through the junction, which leads to increased resistivity. The total resistance of the samples measured using the four probe technique is 1 and 400 kΩ for TiN and TiN_0.76 respectively. In both type of compounds the grain and grain boundary resistances are of the order of MΩ. The grain and grain boundaries are connected in a manner that causes the total resistivity to be lower than the local resistivity.     

Grain-size dependence of the mechanical properties of an age-hardening Fe-1 % Cu-alloy

The contribution of grain size and precipitation hardening to the yield stress and other mechanical properties was investigated. An alloy of iron with 1 % copper was prepared as supersaturated solid solution with grain sizes between 12 and 140 μm. By ageingat 500 and 600° C different precipitation hardening conditions were produced. For small particle sizes an additive behaviour of grain-boundary and precipitation hardening was found (particle radiusr < 50 Å). For large particle sizes the yield stress is independent of grain size (r > 150 Å). A transition is found for intermediate particle sizes with grain size dependence for small and independence for large grain sizes (50 Å <r < 150 Å). The effect of grain boundaries and particles on the formation and motion of dislocations is used to explain this behaviour.     

Numerical simulation of equiaxed grain formation in weld solidification

Numerical simulation is developed on grain structure development during weld solidification of steel. Monte Carlo (MC) method is applied to the simulation of nucleation and growth of solid, being combined with finite difference calculation of heat conduction and solute diffusion. Based on the experimental result that titanium nitride (TiN) works as a nucleating agent of equiaxed grain formation, given number density of TiN is allocated to MC cells randomly and instantaneous nucleation is assumed to occur when the melt undercooling of the cell exceeds a given critical level. It is found that the simulation can reproduce the weld solidification by reflecting the effect of TiN as a nucleating agent and the effect of soluble titanium that increases melt undercooling. Those effects have been recognized only as combined effects in previous experimental studies, but through the present simulation, they are first investigated in a separate manner. The simulation results indicate that equiaxed grain formation is promoted not only by a nucleating agent, TiN, but also by melt undercooling increased by soluble titanium and that both are requisite to bring about the columnar-to-equiaxed transition (CET). The results are discussed in connection with theoretical models on CET and confirm the rationality of the models.

© 2003 Elsevier Science Ltd. All rights reserved.     

Influence of grain size on arrest of a dynamically propagating cleavage crack in ferritic steels—micromechanics

Cleavage fracture in ferritic steels is controlled by several critical steps. First a microcrack must nucleate, grow and overcome barriers, such as grain boundaries. The latter is examined here by use of a periodic, axisymmetric model representing two grains. A microcrack nucleated at the center in one grain is driven by a constant remotely applied stress towards the second grain. The cleavage planes of the grain in which the microcrack is nucleated coincide with the principal loading direction. In the adjacent grain, due to misalignment in possible cleavage planes, the propagation direction changes and separation occurs in mixed mode, involving both normal and shear separations. The temperature dependence of the mechanical properties of the material is accounted for by use of a temperature dependent elasto viscoplastic material model. The largest grain size that can arrest a rapidly propagating microcrack is defined as the critical grain size. The effects of stress state and temperature on the critical grain size are examined. The influence of mismatch in lattice orientation between two adjacent grains in terms of a tilt angle is both qualitatively and quantitatively described. It is shown that the critical grain size is influenced by plastic geometry change and prestraining, which depend on the applied stress state. The results also show that a microcrack can be arrested in an adjacent grain under specific conditions.